Person A opens an event to edit it. Then the phone rings and they get into a five-minute conversation while the edit dialogue box is open. Meanwhile, person B opens the same event in edit mode. *problem - this is allowed*. Person B makes their changes and saves the event. They refresh their browser and see that the changes are there, so they leave happy. Person A hangs up the phone and finally makes their changes to the event. They save it successfully, and by doing so, clobber the changes made by person B. Person B is very unhappy because now their changes have disappeared, and is grumpy for the rest of the day, much to the chagrin of their co-workers.

(Again, apologies if this has been fixed in subsequent releases - hope so, because then it's a great argument to upgrade)